1. Field of the Invention
The present invention relates to thermal transfer printing, and more particularly, to an apparatus for temporarily locking the ribbon hubs of a thermal transfer printer to permit replacement of a rotating ribbon roll.
2. Description of Related Art
In the field of bar code symbology, vertical bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. The bar codes are typically printed onto paper substrate labels having an adhesive backing layer that enables the labels to be affixed to objects to be identified. To read the bar code, the bars and spaces are scanned by a light source, such as a laser. Since the bars and spaces have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the laser light that reflects from the bar code. In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking or blurring of the bar code. At the same time, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process.
In view of these demanding printing requirements, bar codes are often printed using thermal transfer printing techniques. In thermal transfer printing, a label sheet of the print media is drawn between a platen and a thermal print head. A thermally active ink ribbon is drawn along in parallel with the label sheet between the platen and the thermal print head. The thermal print head has linearly disposed printing elements that extend across a width dimension of the label sheet. The printing elements are individually activated in accordance with instructions from a controller. As each printing element is activated, the thermally active chemical of the ribbon activates at the location of the particular printing element to transfer ink to the printed area of the label sheet. The label sheet is continuously drawn through the region between the platen and the thermal print head, and in so doing, the bar code is printed onto the label as it passes through the region. Other images, such as text characters, can be printed in the same manner.
The thermal transfer printer includes a mechanism for transporting the ribbon from a supply hub to the print region. It is desirable within the art to increase the rate at which the labels are printed. At the same time, it is also desirable to increase the overall width of the label (e.g., up to seven inches). Since the ribbon must be at least as wide as the print media, the increasing media size has driven a corresponding increase in size and weight of the ribbon roll (e.g., up to ten pounds). As a result, it is increasingly difficult to install a replacement ribbon roll onto its associated supply hub.
Typically, the ribbon supply hub is disposed within a ribbon transporting assembly of a cage portion of the printer. The cage portion can be pivoted upward to expose the ribbon supply hub as well as the media supply, enabling an operator to service the printer and replace the ribbon and/or media as required. The relatively heavy motors that drive the supply and take-up hubs disengage from the hubs when the cage is pivoted upward. The supply and take-up hubs have a key that mates with an associated notch of the ribbon roll core when the roll is oriented properly. The operator rotates the ribbon roll until the key engages the notch, which permits the roll to seat properly on the supply hub.
A vexing problem often experienced by operators of thermal transfer printers is the difficulty in getting the notch to mate properly with the supply hub key while attempting to replace the ribbon roll. With the motors disengaged from the supply and take-up hubs, the hubs rotate freely in the absence of rotational friction ordinarily provided by the motors. As the ribbon roll is rotated manually by the operator in an attempt to mate the notch and key, the hub is caused to rotate cooperatively with the ribbon roll, and thus, the notch never engages the hub key. The weight of the ribbon roll resting on the hub contributes to the difficulty in manipulating the roll into a proper position. To overcome this problem, the operator will typically remove and reinstall the ribbon roll onto the hub repeatedly until the notch and hub key eventually come into alignment.
Accordingly, it would be desirable to provide a mechanism for a thermal transfer printer that would hold the ribbon hub in a non-rotating state to facilitate replacement of the ribbon roll. Such a holding mechanism should permit rapid replacement of the ribbon roll without impeding movement of the ribbon hub during normal operation of the printer. At the same time, the mechanism should not overly complicate the printer or substantially increase its production cost.